Dump valves have been constructed so as to be spring-loaded to the closed position. In operation, this valve must be held open to permit the passage of the system fluid. A second type of dump valve which is not spring-loaded is opened by rotation. Although this valve need not be held open to permit passage of the system fluid, it must be manually closed before the system is re-pressurized.
In commercial aircraft many standby systems are operated by air or hydraulic pressure. In one system, a dump valve is used to release air under approximately 3000 psi pressure from a cylinder to operate an air motor to open the passenger exit doors. Due to the conditions under which the standby system is used to open the passenger exit doors, it is desirable to have the dump valve open and remain open until the system is substantially exhausted without the need for a crew member to hold the dump valve open. Accordingly, the second type of dump valve has been employed to release the air under pressure to operate the passenger exit doors. As described above, the dump valves are opened and remain open without further effort on the crew member's part. When the valve is to be closed and the system is to be recharged, the valve must be manually re-cycled (closed). In commercial aircraft the standby systems and valves are placed behind decorative paneling. It is expensive and time-consuming to remove the panels and re-arm the dump valve. To avoid the necessity of removing the paneling and re-arming the dump valve, it is desirable that the valve automatically close (re-cycle) so all that is necessary is to recharge the air to 3000 psi in the cylinder.